In wireless and ad-hoc communication networks, time synchronization is often necessary in order for two communication devices to communicate successfully. For channel access using time division multiple access (TDMA), accurate slot alignment requires time of day (TOD) synchronization. For military communications, synchronization is especially important because of the requirements of communication security.
Synchronization schemes depend upon the transmission of timing signals for clock alignments. These timing signals could contain timing information in the fraction microsecond range for accuracy. Global positioning system (GPS) signals can be used to establish TOD synchronization. For a communication device with GPS, the GPS timing signal sent internally at the GPS 1 pulse per second (pps) rate can be used to align the local real time clock (RTC) to the GPS clock. If every communication device in the network has GPS, TOD synchronization is straightforward when the GPS provides periodic time references. However, under real-world operating conditions, this is often not the case, and some mechanisms must be provided to align the clocks of different communication devices. For a communication device without GPS, the communication device has to depend upon the reception of timing signals transmitted from another node to synchronize the clock.
Time synchronization algorithms can be classified as pair-wise and/or global. In pair-wise synchronization, synchronization is achieved between pairs of nodes. In global synchronization, all the nodes in the network must have clock synchronization. Algorithms for global synchronization can be further classified into two categories: level-based synchronization and diffuse-based synchronization. In a level-based synchronization, the network is organized into a level hierarchy and clock synchronization is established level by level from the source to all the nodes in the lower levels. The level architecture could be a tree or cluster depending upon the network formation methods and the source is normally either a cluster head or a root node. In a diffuse-based synchronization, no particular network architecture is assumed to spread the synchronization information to the entire network.
Many existing synchronization schemes are designed to set up synchronization for all the nodes in a network (i.e., synchronization acquisition). However, few, if any, synchronization schemes have been proposed that focus on the problem of maintaining synchronization once it has been established (i.e., synchronization “tracking”). Additionally, there are few, if any, discussions about the detailed signaling structures for the timing signals used for different synchronization schemes.
Normally, timing signals are sent using a different transmission scheme so as not to interfere with the regular data being exchanged between communication devices. For example, timing signals can be sent using different frequencies. In an acquisition mode, communication bandwidth is not usually wasted, since normal data communication has not started and the receiver can afford to search for timing signals. Once the data communication begins and synchronization is in a tracking mode, it is a problem if the receivers do not know when to tune the communication device to the correct frequency to receive the timing signals. Thus, valuable data bandwidth can be wasted in the tracking mode waiting for the receiver to search for the timing signals, resulting in reduced throughput for data communications.
Another shortcoming of current TOD synchronization schemes is the inability to maintain time synchronization in a tracking mode after an initial synchronization in an acquisition mode. In acquisition modes, nodes search for the timing signals to align their clocks. If the timing signal is received and if at least one neighbor is found, the nodes switch to the tracking mode. While in the tracking mode, initial clock alignments cannot guarantee future clock alignments because of clock drift, and as a result, the network nodes need to search for timing signals for clock alignments. Thus, network nodes sometimes have difficulty maintaining time synchronization in the tracking mode.
In data communications using carrier sense multiple access with collision avoidance (CSMA/CA) protocols, the exchanges of request to send (RTS) signals and clear to send (CTS) signals before the start of a message transmission can be used to estimate clock drift.
In an ad-hoc network, neighbor discovery is crucial, and as a result, one-way transmissions of hello RTS (HRTS) signals from each node is often necessary. To compensate for the problems arising during infrequent PTP data traffic, the one-way transmissions of HRTS signals can be used to estimate clock drift. However, in doing so, the network assumes that a destination node has knowledge of the internal delays in the source node. This is because the destination node only knows that the HRTS is sent from the beginning time slot of a timing signal. The destination node, however, does not always know the internal delays in the source node, as there can be delays from the beginning time slot of a timing signal to the start of transmission (SOT) of the timing signal. These delays can be due to the processor, the modem, transmission security, propagation delays and other reasons. Thus, inaccurate estimations of time delays could lead to problems in achieving time synchronization.